This invention relates generally to electric motors and more particularly, to methods for controlling the torque produced in electric motors.
Electric motors typically include a stationary outer portion, or housing, with a bore therethrough, a stator mounted in the housing, a rotor rotatably mounted in the housing, and a rotor core having a bore therethrough and a substantially straight shaft extending through the rotor core bore. The shaft is rotatably supported by a set of bearings and rotates utilizing magnetic fields. The stator includes a bore therethrough, stator coils and stator windings, with each stator coil wound around a respective stator winding. The rotor extends through the stator bore and includes a set of windings. Electrical current flows through the stator coil in the respective stator winding in a time sequential manner, which generates a stator magnetic field that repels/attracts a rotor magnetic field. The electrical current flowing through the stator constantly changes in time and direction, resulting in a constantly changing stator magnetic field. Due to the changing current direction and a resulting rotating stator magnetic field of constant magnitude, the rotor is caused to rotate and generate mechanical energy.
Many electric motors are fabricated to operate at multiple speeds. One way in which multiple speeds are obtained is by providing additional windings, connected a variety of ways in the circuit. Sometimes windings with a diminished number of turns of wire at a pole are created by xe2x80x9ctappingxe2x80x9d the existing windings. The tapping of existing windings disconnects part of a winding from the rest of the circuit. However, depending on the location of the tap, the number of turns of wire tapped, and the remaining turns of wire active in the machine, an electromagnetic imbalance in the normal near-sinusoidal flux distribution can be created because a number of adjacent poles are not energized. Even harmonics and sub-harmonics are added to the decomposition of flux distribution when poles are tapped adding a new forcing function which drives vibration and adds losses, reduces the effect of the electromagnetic flux, and results in less torque per ampere of stator current.
One method for overcoming an electromagnetic imbalance is to provide a tap only between full complements of poles. This method is similar to electrically providing additional, distinct windings, and has the manufacturing benefit, of depending on fewer pieces of machinery to create the final set of windings, although unenergized poles are still present.
It would therefore be desirable to provide another method of controlling the speed of an electric motor that does not suffer from electrical imbalance and is distinct electro-magnetically from conventional methods such as the tapping between full complements of poles and therefore eliminating even subharmonics of the decomposition of the flux distribution.
In an exemplary embodiment, a method of adjusting speed and torque of a dynamoelectric machine is disclosed. The machine includes a main winding, and a divided winding each configured to generate a plurality of poles. The main winding includes winding at each pole which are alternately wound, the divided winding includes winding which are consecutively wound. The method includes the steps of energizing the main winding and controlling the amount of electromagnetic flux at each pole. The amount of electromagnetic flux at each pole is controlled by energizing the divided winding.